please let us know if any method is available to speed up the CTAS command. As delete command not free the OS Disk space and no option to drop past day records. Hence need to execute the CTAS command on daily basis.

Only text datatypes are dictionary encoded, so yes, you can share just string encoded fields.

The strings are stored on a different file than the table data. It’s encoded as an integer (or smallint, tinyint depending the number of distinct values expected in the column), and it’s possible to share a dictionary between different tables.
Doing that you will save disk space and you will speed up the IAS because it’s likely you haven’t to decode and re-encode the string from the source table (I’m not sure of that).
The tradeoff using the shared dictionaries is that the strings are varying a lot between data (e.g., you have something like a preformatted datetime/timestamp stored as dictionary encoded strings), you won’t recover the space of unused ones. So before using this approach, checks if it’s worth the time saved.

Another idea could be to use IAS with shared dictionaries daily and doing a complete rebuild weekly or monthly.

I did some test and yes, you have the data into the new table and the dictionary will survive the drop because is used by the new table